There is a well known fact that the joints of humans as well as animals deteriorate over time. Sometimes deterioration occurs because of disease due to aging and sometimes due to a trauma. The knee joint is perhaps the one joint that has given people the most trouble over the years because of its essential function to the mobility of humans. Thus, quite naturally, a great deal of research has gone into the development of replacement knees. Today, because of this widespread research, a significant portion of our population is again mobile and relatively free of pain. This great step forward in medical research has not been without its difficulties. Primarily because of the tremendous stresses placed on the knee joint, there continues to be a need for improvement to eliminate the last vestiges of pain and suffering.
Prosthetic surgery involving the excision and removal of deteriorated and diseased bone tissue in knee and hip joints has now become quite common. Typically, artificial members of plastic and/or metal compatible with the human body are substituted for the removed natural bone segments and anchored to the remaining bone structure.
The bearing surfaces of the knee joint are especially vulnerable to stress, arthritic and other disease induced deterioration. Prosthetic correction is necessary when the surfaces become so damaged that other less drastic techniques have little or no prospect of success.
In a healthy knee, the lower leg bone, called the tibia, has at its upper end a pair of concave surfaces. These concave surfaces meet with condyles which are formed in the lower end of the femur, which is the upper leg bone. This meeting is protected by the patella (knee cap). Thus, in a knee replacement operation, the upper end of the tibia is removed as is the lower end of the femur, and these bones are replaced with prostheses, which are designed to operate together.
In the past, there have been two major types of knee prostheses: hinged and non-hinged. In one form, the knee was resected and replaced by a metal hinged-type device with deep penetration into the remaining femoral and tibial bone structure by means of flared and thick stems. The range of movement was limited and patients were seldom able to flex the knee beyond 90.degree. . Moreover, implantation required the removal of a significant amount of the bone with a shortening of the limb if for any reason the prosthesis was later removed.
More recent prostheses using a different approach attempt to structurally resurface both of the articulating surfaces of the knee to provide a non-hinged type prosthesis. Such prostheses seek to remedy the failure of hinged devices, and involve two components which are respectively connected with the femur and tibia and held in engagement by the muscles and ligaments to produce a more lifelike situation.
The non-hinged knee prostheses must contend with the particulars of the human knee joint. The tibia is situated at the front (anterior) and inner (medial) side of the lower leg and, except for the femur, is the longest and largest bone in the human skeleton. It is prismoid in form, expanded above, where it enters into the knee joint. The head of the tibia is large and expanded on each side into two eminences called the tuberosities. The tops of these present two smooth concave surfaces which articulate within the knee with the condyles of the femur. The medial condyle is more prominent anteriorly and broader both in the anterior-posterior (front-rear) and transverse diameters than is the outside (lateral) condyle. Accordingly, the lateral top articular surface of the tibia is longer, deeper and narrower than the medial surface of the tibia so as to articulate with the lateral condyle. The medial surface is broader and more circular, concave from side to side, to articulate with the medial condyle. The anterior surfaces of the tuberosities are continuous with one another, forming a single large surface which is somewhat flattened. Posteriorly the tuberosities are separated from each other by a shallow depression for attachment of ligaments. The inner (medial) tuberosity presents posteriorly a deep transverse groove for the insertion of the posterior cruciate ligament (PCL).
In the past, tibial prostheses (commonly called baseplates because they fit beneath the condyles of the femur) were manufactured with total symmetry for use with both left and right knees which, as was discussed above, are not symmetrical. Although the symmetrical components were interchangeable between the right or left tibia, there were problems with the baseplate overhanging the lateral tibial bone surface or undersized on the medial tibial bone surface. The result was a compromise in the fit of the prosthesis. In response thereto, asymmetric tibial prostheses (baseplates) were developed to more closely approximate the natural tibial anatomy. The problem with such asymmetric tibial prostheses is that they require asymmetric, or knee specific removable tibial baseplate inserts (the concave bearing surface which actually contacts the femoral condyles) necessitating the need for a hospital to maintain an inventory of left and right knee specific inserts.
U.S. Pat. No. 4,963,152 provides a partial solution to the problems associated with symmetrical tibial prostheses. The patent discloses an asymmetrical tibial prosthesis whereby asymmetry is created by having the outer radius which describes the medial condyle slightly greater than the outer radius describing the lateral condyle. However, the baseplate is symmetrical about the medial-lateral centerline, allowing it to be used on either left or right tibia by rotating the baseplate 180.degree. about the centerline. The insert, which is designed to mate with the baseplate, is also symmetrical about the medial-lateral centerline. The disadvantage of such a tibial prosthesis is that it must include an anterior as well as a posterior relief notch for the PCL in order to allow the baseplate to be reversed and thus usable on either the left or right knee. The "extra" notch, along with the symmetrical geometry of the baseplate about the medial-lateral centerline, does not provide the best coverage of the prosthesis upon the head of the tibia.
In order to provide the best coverage, the baseplate should be formed with only one notch for the PCL and thus cannot be symmetrical about either the medial-lateral centerline or the anterior-posterior centerline. However, such a baseplate would require a correspondingly asymmetrical insert under the teachings of the prior art.
Since these inserts may wear out and require replacement prior to the tibial baseplate, it becomes more costly from an administrative standpoint to maintain an inventory of both left and right knee inserts.
What is needed is a low cost symmetrical tibial insert that can be mass-produced so as to lower inventory and replacement costs and which is still usable in conjunction with an asymmetric tibial baseplate.
The object of the present invention is to provide a knee prosthesis having a tibial baseplate specific to the left or right tibia but allowing for the placement of an insert which is symmetrical about its anterior-posterior centerline and thus not left or right knee specific. The knee prosthesis also includes a femoral component having condyles for articulating with the condyle compartments of the symmetrical insert.